The present invention relates to electrical connectors and, more particularly, to techniques for shielding electrical connectors for preventing leakage of radio frequencies therefrom.
Low-resistance electrical continuity in electrical shields has long been a recognized requirement for preventing leakage of radio-frequency interference (RFI) capable of disrupting the performance of external electronic equipment. Cables are frequently shielded with metallic braid and/or metallic foil forming a cylinder about signal-carrying conductors. Openings in the braid are kept small enough that they are substantially continuous at the frequencies of interest.
A recognized problem in maintaining shielding integrity is found in connectors for connecting the signals on the cables to other equipment or other cables. One type of connector provides an interface between a shielded cable and the pins or holes of a mating connector. The shielding braid or foil on the cable is conventionally trimmed short of the point at which connection with the wires is made in the connector. If no provision is made to prevent it, RFI may escape from the unshielded exposed wires within the connector.
U S. Pat. Nos. 3,322,885 and 3,375,483 disclose a conductive plastic material cast in a connector contacting the cable shield and the interior of the metallic casing or hood of the connector for producing electrical continuity therebetween. Although this technique may be effective for providing electrical shielding continuity between the braid or foil on the conductors and the connector casing, it fails to lend itself to assembly by an unsophisticated consumer. In addition, even in a commercial cable assembly application, the requirement for one or two plastic casting operations makes such techniques too complicated compared to available alternatives.
U.S. Pat. No. 3,424,853 discloses a mesh gasket ring, U.S. Pat. No. 3,739,076 discloses a coil spring in the form of a ring, and U.S. Pat. No. 3,830,957 discloses a resilient ring covered with a braid. All of these patents are adapted to maintaining shielding integrity in a cylindrical connector. The conductive element is compressed into circular contact with the shield braid or other cylindrical object by an annular beveled surface. None of these disclosures appear relevant to providing shielding integrity in flat connectors of common use in connecting computer equipment and peripherals.
In U.S. Pat. No. 4,575,174, of common assignee with the present invention, an electrical connector hood is disclosed having an integral strain relief. A strain relief tab, integrally formed with each half of the hood extends toward the cable passing therebetween to transfer forces from the cable to the hood without permitting such forces to reach soldered connections in the connector. This invention is concerned with strain relief and not with shielding continuity. Indeed, the preferred embodiment of the invention is plastic. Shield braid on a cable is neither illustrated nor described.
Communication between computer equipment and peripherals conventionally employ serial or parallel data transfer on shielded cables. Parallel data cables conventionally employ 50-pin connectors. Serial data cables may employ from as few as two to as many as 50 or more conductors connected to a male or female connector. Common serial connectors are identified by the maximum number of conductors they are capable of handling. For example, a DB-9 connector is capable of connecting up to 9 conductors. A DB-25 connector is capable of connecting up to 25 conductors. Due to the wide distribution and use of DB-25 connectors, such a 25-pin connector is chosen for the following description. It will be understood that other connectors fall within the scope of the invention.
A shielding DB-25 connector includes a connector and a hood consisting of mating halves formed of metal or plastic with a conductive coating on a surface thereof. The shield braid or foil is trimmed back from the bared ends of the conductors. The conductors are soldered to appropriate ones of the pins or sockets in the connector. The two halves of the hood are then assembled over the connector and cable end with the cable passing out through an opening formed between the two halves of the hood.
DB-25 connectors are employed for connecting from as few as one or two, to as many as 25, shielded conductors to an equipment or to another cable. The diameter of a cable containing one or two conductors is substantially different from the diameter of a cable containing 25 conductors. In the prior art, the effective diameter of the opening in the hood is varied for different cable diameters in order to achieve both strain relief and electrical continuity between the cable shield and the hood. The variation in effective diameter is conventionally achieved by one of the following techniques: different hood models each having a different entry diameter, each hood shipped with several resilient grommets having outside dimensions fitting an enlarged opening in the hood and varying inside dimensions each suitable for a small range of cable diameters, and a cable clamp adjustable by screw or ratchet to clasp the cable.
In the prior-art technique employing different opening diameters, the shield is compressed between the halves of the hood for attaining electrical continuity therebetween. This technique has the disadvantage that a cable assembler must carry a number of different hood models in stock in order to accommodate any number of conductors in the cable.
In the prior-art technique employing grommet adapters, four or five grommet sizes are provided with each hood, of which only one is used. The remainder are scrap. This represents a substantial waste of resources. In addition, the grommet must be slipped onto the cable before affixing the conductors to the connector. If it is later discovered that the wrong size of grommet was chosen, the partially assembled connector must be either discarded or disassembled. Further, in order to attain electrical continuity, the shield braid or foil must be folded back along the outside of the grommet before the halves of the hood are assembled thereon. It has been found very difficult to perform this folding, particularly for shield braid. The difficulty is found to be particularly severe in a custom cable assembly operation where an operator is expected to assemble many cables in a working day.
Finally, the clamping techniques of the prior art have not addressed the RFI problem. Generally, when clamping a cable having a small number of conductors, a substantial opening exists between a clamping member and the opening in the hood. RFI may propagate through such an opening.